I recently purchased a used RV, and the black tank sensors will show 25% full, then jump to 100% full, even though I know there's no way there's that much waste in it. My belief is that there is some residue (maybe paper) that is clinging to the 1/2, 3/4, and Full sensor, which is giving a false reading once it gets a little wet. I've used the camper, filling the tanks, and even then after flushing, the problem persists. What can I do to clean the sensors and stop this false reading? Are there additives I could use, let sit so it can dissove, then wand the tank, ice cubes, etc? Thanks in advance for any suggestions.